I read a bunch about this on a couple of places, including some of the actual law. What I got from it, and I'm not a lawyer, was basically this:
If you take a shotgun that's only ever been pistol grip AND put that pistol grip on it AND keep it over 26" overall you can cut the barrel down to about 14". Id had something to do with the fact that PG shotguns are sold as firearms not long guns (it's why you have to be 21 to get one) so it's a work around to the 18" barrel requirement, and it's not a registered AOW/SBS/SBR no stamp needed.
HOWEVER: this still doesn't fly in some states, and I don't think anyone has been arrested yet for having it. I thought it would make a good truck gun idea, but realistically an AR pistol is a better fit. I also didn't want to be the first one caught with this.
Do a bunch of reading before you decide if it's right for you. They sell full kits for the moss berg with 14" barrel and tube, etc, that would let you build this for about $210 on top of the base shotgun.